Recently, there has been a great deal of interest in the use of a compacting press to manufacture parts from iron powders and other powders (such as non-ferrous powders, ceramic powders, and the like) where each particle of powder is coated with a very thin layer of polymer. If these powders are compacted at temperatures ranging from about 450.degree. F. to about 550.degree. F., the polymer materials "set" and the compacted parts have sufficient strength to eliminate the need for sintering after the compacting operation, although sintering can be practiced if desired.
In conventional compacting operations utilizing uncoated iron powder or the like, the powder is directed from a storage hopper by means of a flexible hose to the hopper of a delivery shuttle by which the powder is shifted to a die cavity in a measured amount. Difficulties are encountered, however, when attempting to use the same sort of delivery system for polymer coated powder wherein the powder is delivered to a heated die and held for a time sufficient to heat the powder to its desired temperature before the compacting operation. First of all, this approach is very time consuming. In addition, however, it causes localized over-heating where the powder is in contact with the heated die. The polymer coating acts as a temperature insulator, and those coated particles in the interior of the mass are slow to heat, while the polymer-coated particles adjacent the heated die "set" before the interior particles are hot enough to compact.
Attempts have also been made to heat the coated powder by external means to a temperature just below the "set" temperature prior to loading the coated powder into the die cavity. It is characteristic of the powder that it starts to coagulate and become "tacky" at a temperature of about 350.degree. F. Since the usual production compacting method requires accurate gravity-controlled filling of the powder into the die cavity from a delivery shuttle, any coagulation or tackiness of the coated powder causes variations in the amount of powder that actually is deposited in the die cavity.
The present invention is directed to a coated powder delivery system which will overcome the above-noted problems. The delivery system of the present invention will accept a variety of polymer coated powders of metal or other materials. Each coated particle is heated to a temperature just below the coagulation point. The heated powders are accurately fed into a heated die cavity on a conventional compacting press. The remaining increase in temperature to the "set" point is then rapidly achieved during a standard compacting cycle by a combination of the heated tooling and the energy imparted during the actual compacting stroke.